Furnace and a method of controlling a furnace

ABSTRACT

The invention provides a furnace having a frozen lining inbetween a furnace lining and a furnace charge, the furnace having means to control the operation of the furnace, including means to measure the temperature in a wall of the furnace adjacent the frozen lining, and to estimate the thickness of the frozen lining as a function of the temperature in the wall, and means to control the rate of heat production in the furnace to urge a thickness of the frozen lining towards a predetermined value. Variables including side wall thermocouple measurements, gas plant instrument measurements, cooling system measurements and electrical, in-feed and chemical composition recordings are monitored, analysed and manipulated. There is also provided means to estimate a future furnace charge composition, perform chemistry control, estimate a material balance of the furnace, and perform inventory control over the furnace.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to application no. ZA 2001/1817,the entire contents of which is expressly incorporated herein byreference thereto.

FIELD OF THE INVENTION

[0002] The present invention relates to a furnace and a method ofcontrolling a furnace.

BACKGROUND OF THE INVENTION

[0003] THIS invention relates to a furnace that can be used for theproduction of a metal value from a metal value bearing material andspecifically to such a furnace where the refractory lining is protectedby a frozen lining.

[0004] In processes for the production of metals from metal bearingores, furnaces frequently form an indispensable part of the process. Theore is subjected to heat and certain reagents to unlock the metalcontained therein and to transform it to a form from where it can beworked further.

[0005] Because of the high heat necessary for most processes and theneed to contain the heat energy and the charge, furnaces almost alwaysneed an insulating lining known as a refractory lining.

[0006] Refractory linings form a high cost component in the productionprocess due to their specialised nature, and the downtime needed toinstall a refractory lining also contributes to the cost factor.

[0007] There is therefore a need to maintain a refractory lining for aslong as possible. Refractory linings wear away by a number of differentmechanism including mechanical thermal and chemical wear.

[0008] Mechanical wear occurs through abrasion of charged materialagainst the refractory lining material, such as may occur duringcharging of a furnace with hard material.

[0009] Thermal wear occurs when the temperature of the refractory liningmaterial rises above a certain refractory-specific safe limit. Abovesuch a temperature, the refractory lining material loses its strengthand may start to dissolve into the charge.

[0010] Chemical wear occurs when the refractory lining is exposed tochemical compositions that tend to remove certain elements of compoundsfrom the refractory lining material, thereby weakening its structure.This frequently occurs through what is commonly known as a slag-attack,where a layer of slag on top of a charge of liquid steel will attack therefractory lining material of a steel-making furnace.

[0011] One way of preventing or reducing refractory lining wear is byestablishing and maintaining a layer of frozen charge between the chargeand the refractory to serve as a barrier against mechanical, thermal andchemical wear.

SUMMARY OF THE INVENTION

[0012] In accordance with this invention there is provided for a furnacehaving a furnace lining and a charge therein to have a frozen lining atleast partly between the furnace lining and the charge, and for means tocontrol the operation of the furnace, the control means including meansto measure the temperature in a wall of the furnace adjacent the frozenlining, and means to estimate the thickness of the frozen lining as afunction of the temperature in the wall and means to control the rate ofheat production in the furnace to urge a thickness of the frozen liningtowards a predetermined reference value.

[0013] There is also provided for the means to control the rate of heatproduction in the furnace to include control over the rate of additionof carbonaceous reductant to the furnace, for the rate of heatproduction to increase with an increase in the rate of addition ofcarbonaceous reductant to the furnace to thereby urge the thickness ofthe frozen lining to decrease; or for the rate of heat production todecrease with a decrease in the rate of addition of carbonaceousreductant to the furnace to thereby urge thickness of the frozen liningto increase.

[0014] There is also provided for the control means to include means tomeasure furnace gas plant variables means to measure furnace coolingsystem variables, means to measure furnace in-feed variables, means tomeasure furnace electrical system variables, and means to measurefurnace charge chemical composition variables.

[0015] There is also provided for a process of error detection andvalidation to be conducted on the measurements, for the process of errordetection and validation to include analysis of the range of themeasurements and the rate of change of the measurements to validate themeasurements, and for invalid measurements to be replaced bypre-recorded measurements according to a set of logical rules.

[0016] The invention further provides for means to estimate the frozenlining thickness and hot face temperature as a function of the walltemperature measurements and gas plant measurements.

[0017] There is also provided for means to estimate heat losses in thefurnace as a function of estimated frozen lining thickness and hot facetemperatures, the gas plant measurements, and the cooling systemmeasurements.

[0018] There is also provided for means to measure sensible heat changesof spray cooled roof panels, spray cooled off gas ducts, film cooledshell panels, air cooled hearth panels, hot gasses and dust, and chargeremoved from the furnace.

[0019] There is also provided for means to estimate heat losses in thefurnace as a function of the estimated frozen lining thickness and hotface temperatures, the gas plant measurements, the cooling systemmeasurements, and measured sensible heat changes.

[0020] The invention also provides for means to estimate a materialbalance of the furnace as a function of the estimated frozen liningthickness and hot face temperatures, the gas plant measurements, thein-feed measurements, the electrical system measurements, and thefurnace charge chemical composition measurements.

[0021] There is further provided for means to perform inventory controlover the furnace using the material balance of the furnace.

[0022] The invention further provides for means to estimate a futurefurnace charge chemical composition as a function of the estimatedfrozen lining thickness and hot face temperatures, the estimated heatlosses, and the estimated material balance.

[0023] There is also provided for means to perform chemistry control ofthe furnace using the estimated material balance, the estimated futurefurnace charge chemical composition, the in-feed measurements, theelectrical system measurements, and the furnace charge chemicalcomposition measurements.

[0024] The invention further provides for start-up control of thefurnace to be performed using the in-feed measurements, the electricalsystem measurements, the furnace charge chemical compositionmeasurements, and the estimated heat losses.

[0025] The invention also provides a method for controlling a frozeninterface between a furnace lining and a charge in the furnace, themethod comprising the steps of:

[0026] (i) establishing the frozen lining;.

[0027] (ii) measuring at least the temperature in a wall of the furnaceadjacent the frozen lining;

[0028] (iii) estimating the thickness of the frozen lining as a functionof the temperature in the wall; and

[0029] (iv) controlling the rate of heat production in the furnace tourge a thickness of the frozen lining towards a predetermined referencevalue.

[0030] There is also provided for step (ii) of the method to includemeasuring gas plant variables, furnace cooling system variables, furnacein-feed variables, furnace electrical system variables, and/or furnacecharge chemical composition variables.

[0031] The invention also provides for the method to include performinga process of error detection and validation on the measurements, theprocess of error detection and validation including the steps of.

[0032] (v) analysing the range of the measurements and the rate ofchange of the measurements;

[0033] (vi) validating the measurements; and

[0034] (vii) replacing invalid measurements by pre-recorded measurementsaccording to a set of logical rules.

[0035] The invention further provides for step (iii) of the method toinclude a step of estimating the thickness of the frozen lining as afunction of the temperature in the wall and the furnace gas plantmeasurements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 is a schematic representation of the logic of a controlprocess for a furnace according to the invention.

[0037]FIG. 2 is a schematic representation of the feed-back used in thecontrol process.

[0038]FIG. 3 is a schematic representation of the multi-use of inputinformation in the chemistry control.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

[0039] A process control layout for a furnace (not shown) with arefractory lining and a charge is generally depicted by referencenumeral 1 in FIG. 1. The furnace has a frozen lining (not shown) betweenthe refractory lining and the charge.

[0040] Instrument readings from the furnace include sidewallthermocouple measurements (2), gas plant instrument measurements (3),cooling system instrument measurements (4), and in-feed, electricalsystem and charge chemical composition measurements (5).

[0041] A process of error detection and validation (6) is conducted onthe instrument measurements. The process includes analysis of the rangeof the measurements and the rate of change of the measurements tovalidate the measurements, the replacement of invalid measurements byprerecorded measurements according to a set of logical rules.

[0042] Once the measurements have been validated, or if invalid,replaced by substitute values, the measurements are utilised for thecontrol of the process.

[0043] The frozen lining thickness and hot face temperatures areestimated (7) as a function of the sidewall thermocouple measurements(2) and gas plant instrument measurements (3). The estimated frozenlining thickness and hot face temperatures (7) are used to control thefrozen lining thickness (11).

[0044] The heat losses in the furnace is estimated (8) as a function ofthe estimated frozen lining thickness and hot face temperatures (7), thegas plant measurements (3), and the cooling system measurements (4).

[0045] The sensible heat changes of spray cooled roof panels (notshown), spray cooled off gas ducts (not shown), film cooled shell panels(not shown), air cooled hearth panels (not shown), hot gasses and dust(not shown), and charge removed from the furnace (not shown) is alsomeasured and used in the determination of the heat losses in the furnace(8).

[0046] A material balance of the furnace (not shown) is determined andis used in the inventory control (9) of the furnace. The materialbalance (not shown) is determined as a function of the estimated frozenlining thickness and hot face temperatures (7), the gas plantmeasurements (3), the in-feed measurements, electrical systemmeasurements, and furnace charge chemical composition measurements (5).

[0047] The future furnace charge chemical composition (10) is estimatedas a function of the estimated frozen lining thickness and hot facetemperatures (7), the estimated heat losses (8), and the estimatedmaterial balance (not shown). The estimated future furnace chargechemical composition (10), together with the estimated material balance(not shown), the in-feed measurements, electrical system measurements,and furnace charge chemical composition measurements (5) are used toperform chemistry control over the furnace (12).

[0048] Start-up control over the furnace (13) is performed using thein-feed measurements, the electrical system measurements, the furnacecharge chemical composition measurements (5) and the estimated heatlosses (8).

Ilmenite Smelting Furnace

[0049] The process control as described above is used for the control ofan ilmenite smelting process in a DC arc furnace. Ilmenite mineral sandis smelted using anthracite as a reductant in the furnace. The furnaceis refractory lined with magnesite bricks. Cold ilmenite, preheatedilmenite and anthracite are fed into the furnace through a hollowelectrode. High titania slag and metallic iron are periodically tappedfrom the furnace. Hot gas containing dust is removed from the furnacethrough a single off gas duct where it is subsequently cleaned in a gasscrubbing plant. A film of flowing water cools the furnace shell. Theroof panels and off gas panels are spray cooled and the hearth of thefurnace is air-cooled.

[0050] The furnace frozen lining and chemistry are controlled by theamount of energy and carbon reductant input.

Gross Error Detection and Validation

[0051] Plant instruments can fall or drift, thereby giving invalid orinaccurate readings. This would make any calculation or model useless.For this reason, all raw data readings used by the control system gothrough an error detection and data validation process. The quality ofthe readings is marked as either good or bad. The model components aremarked as either enabled or disabled based, on the status of their inputtags. In the gross error detection, the range of the reading and itsrate of change are checked for abnormalities. The data is validated byeither a set of logical rules or neural network models, where theimportant data that is bad for some reason can be reconstructed ifnecessary.

Temperature Profiles and Frozen Lining Thickness Estimations

[0052] Dual sidewall thermocouples are used to read the temperature ofthe sidewalls. Together with knowledge of the thermal conductivity ofthe frozen lining and the refractory lining, an internal nodecalculation is performed to determine the temperature at any pointbetween a sidewall thermocouple and the hot face, which is the interfacebetween the refractory and frozen lining. This information is used tocalculate the hot face temperature and frozen lining thickness.

[0053] The value of the frozen lining thickness is used in the frozenlining thickness control. This value is of more use in the frozen liningcontrol than just the thermocouple readings, because it takes non-steadystate conditions and time lapses between thermocouple readings and thefrozen lining thickness into account.

Inventory Control

[0054] The total amount of material, including the dust losses, andpower added to the furnace between taps is determined for use ininventory control, The analyses of certain elements in the feedmaterials, slag and iron are used in the material balance to determinethe relative amounts of slag and iron produced The amount of frozen slagis taken into account via the frozen lining thickness calculation. Bathheights are calculated through the relationship between mass and volume.The relative amounts of slag and iron to be tapped are then determinedusing the heights of the tap holes as reference points. During theaddition of electrodes, sounding measurements are taken through a hollowelectrode. The actual measurements of the slag and iron bath heights areused to “zero” the control process calculation on almost a daily basis.

Heat Loss Calculations

[0055] Heat lost through the cooling system and exiting streams from thefurnace is calculated by means of the sensible heat gain or loss of thecooling medium and exiting stream. The spray cooled roof panels, spraycooled off gas ducts, film cooled shelf panels, air cooled hearthpanels, hot gasses and dust, and charge removed from the furnace are allused to take readings for the heat loss calculations.

Chemistry Predictors

[0056] Neural network models with high correlation coefficients are usedto predict the current % TiO₂ in the slag, % C in the iron and % Fe₂O₃in the ilmenite, as well as those percentages 2 hours ahead. This datais used for feed forward control in the material and energy balance ofthe decision support module. The neural network models are extensive.There are approximately 42 inputs to each of the iron and slag modelsand 6 to the ilmenite model. Inputs include the data derived from theother modules (frozen lining thickness, inventory control, heat loss).The models auto train as the plant conditions change.

Frozen Lining Control

[0057] The philosophy used in the control of the freeze lining is thatthe frozen lining is viewed as an additional layer of “bricks”. As longas the frozen lining is maintained, the magnesite bricks will remainintact and should not have to be replaced for many years. Themaintenance of even and uniform frozen lining means that the bath sizeis kept constant which makes for better operational control. Tightcontrol of the frozen lining thickness is achieved by making regularchanges to the C reductant addition rates in both the positive (frozenlining getting thinner because of an increased rate of heat production)and negative (frozen lining getting thicker because of a decreased rateof heat production) directions.

Chemistry Control

[0058] The reaction governing the process is given by:

FeTiO₃+C+heat→Fe+TiO₂+CO

[0059] The control objectives are to maintain the % TiO₂ in the slag of86% with minimal deviation and to maintain the freeze lining. This isachieved through manipulation of the C reductant addition rate (AIR,anthracite to ilmenite ratio) and the energy by input (IPR, ilmenite topower ratio). The system is interactive in that both of the manipulatedvariables influence both of the controller variables, as is shown inFIG. 3.

[0060] There are two portions to the control strategy, namely a feedforward portion (ff) and a feed back portion (fb). The feed forwardportion attempts to absorb the disturbance introduced by varying feedmaterial composition (ilmenite and anthracite analyses) and the feedback portion reacts on measurements of the controlled variables (% TiO₂in slag and freeze lining thickness).

[0061] Eff+Efb=Etot (IPR—specific energy, kg ilmenite per (MWh)

[0062] Cff+Cfb=Ctot (AIR—specific carbon, kg anthracite per ton ofilmenite)

[0063] The Eff portion is determined from a hard coded energy balanceand the Cff from a hard coded material balance.

[0064] The Efb and Cfb portions are equivalent to the changes that wereconventionally made by the shift supervisors based on the % TiO₂ in theslag as-tapped and the sidewall thermocouple readings respectively. Inthe decision support system, these portions are determined by a fuzzylogic rule set that was derived from the experiences or operationalstaff and on line tuning.

[0065] The feed back portion consists of two loops, one fast and theother slow, as is shown in FIG. 2.

[0066] The fast loop is run every 15 minutes and uses the estimatedfrozen lining thickness. The slow loop is run after each tap and usesthe % TiO₂ in the slag.

Start-up Module

[0067] During a furnace stoppage, the length of the stoppage and energylost is integrated. A given percentage of the lost energy is thenrecovered through a specified power ramp, IPR and AIR schedule. Once thestart-up module is completed, the system switches back to the chemistryand freeze lining control modules.

[0068] The invention is not limited to the precise constructionaldetails as herein described.

[0069] The applicant believes that the invention is advantageous in thatit provides a furnace with means to control a frozen lining in thefurnace, wherein the bath size is kept constant for better operationalcontrol and the wear on the refractory lining is reduced.

We claim:
 1. A furnace including: a furnace lining and a frozen liningwhich is positioned at least partly between the furnace lining and acharge within the furnace; and control means to control the operation ofthe furnace, the control means including means to measure thetemperature in a wall of the furnace adjacent the frozen lining, meansto estimate the thickness of the frozen lining as a function of thetemperature in the wall, and means to control the rate of heatproduction in the furnace to urge a thickness of the frozen liningtowards a predetermined reference value.
 2. A furnace according to claim1, wherein the means to control the rate of heat production in thefurnace includes control over the rate of addition of carbonaceousreductant to the furnace, so that the rate of heat production isincreased with an increase in the rate of addition of carbonaceousreductant to the furnace to thereby urge the thickness of the frozenlining to decrease, or the rate of heat production is decreased with adecrease in the rate of addition of carbonaceous reductant to thefurnace to thereby urge the thickness of the frozen lining to increase.3. A furnace according to either one of claim 1, wherein the controlmeans further comprises one or more of the following measurement means:means to measure furnace gas plant variables; means to measure furnacecooling system variables; means to measure furnace in-feed variables;means to measure furnace electrical system variables; and means tomeasure furnace charge chemical composition variables.
 4. A furnaceaccording to claim 1, which further comprises a process for conductingerror detection and validation of the measurements, the processcomprising analysis of the range of the measurements and the rate ofchange of the measurements to validate the measurements, and a processfor replacing invalid measurements with pre-recorded measurementsaccording to a set of logical rules.
 5. A furnace according to claim 3,which further comprises means to estimate the frozen lining thicknessand hot face temperatures as a function of the wall temperaturemeasurements and gas plant measurements.
 6. A furnace according to claim3, which further comprises means to estimate heat losses in the furnaceas a function of estimated frozen lining thickness and hot facetemperatures, the gas plant measurements, and the cooling systemmeasurements.
 7. A furnace according to claim 1, which further comprisesmeans to measure sensible heat changes of spray cooled roof panels,spray cooled off gas ducts, film cooled shell panels, air cooled hearthpanels, hot gasses and dust, and/or charge removed from the furnace. 8.A furnace according to claim 7, which further comprises means toestimate heat losses in the furnace as a function of the estimatedfrozen lining thickness and hot face temperatures, the gas plantmeasurements, the cooling system measurements, and measured sensibleheat changes.
 9. A furnace according to claim 3, which further comprisesmeans to estimate a material balance of the furnace as a function of theestimated frozen lining thickness and hot face temperatures, the gasplant measurements, the in-feed measurements, the electrical systemmeasurements, and the furnace charge chemical composition measurements.10. A furnace according to claim 9, which further comprises means toperform inventory control over the furnace using the material balance ofthe furnace.
 11. A furnace according to claim 9, which further comprisesmeans to estimate a future furnace charge chemical composition as afunction of the estimated frozen lining thickness and hot facetemperatures, the estimated heat losses, and the estimated materialbalance.
 12. A furnace according to claim 11, which further comprisesmeans to perform chemistry control of the furnace using the estimatedmaterial balance, the estimated future furnace charge chemicalcomposition, the in-feed measurements, the electrical systemmeasurements, and the furnace charge chemical composition measurements.13. A furnace according to claim 6, which further includes means forcontrolling start-up of the furnace to be performed using the in-feedmeasurements, the electrical system measurements, the furnace chargechemical composition measurements, and the estimated heat losses.
 14. Amethod for controlling a frozen interface in a furnace between a furnacelining and a charge in the furnace, the method comprising the steps of:(i) establishing the frozen lining; (ii) measuring at least thetemperature in a wall of the furnace adjacent the frozen lining; (iii)estimating the thickness of the frozen lining as a function of thetemperature in the wall; and (iv) controlling the rate of heatproduction in the furnace to urge a thickness of the frozen liningtowards a predetermined reference value.
 15. A method according to claim14, wherein the step of measuring at least temperature in the wall ofthe furnace comprises measuring one or more of gas plant variables,furnace cooling system variables, furnace in-feed variables, furnaceelectrical system variables, and furnace charge chemical compositionvariables.
 16. A method according to claim 14, which further comprisesthe step of performing a process of error detection and validation onthe measurements, the process of error detection and validationincluding the steps of: (v) analysing the range of the measurements andthe rate of change of the measurements; (vi) validating themeasurements; and (vii) replacing invalid measurements by pre-recordedmeasurements according to a set of logical rules.
 17. A method accordingto claim 14, wherein the thickness of the frozen lining is estimated asa function of the temperature in the wall and the furnace gas plantmeasurements.